Semiconductor laser diodes have found applications in a wide variety of information handling systems, partly because of their compact size and partly because their technology is compatible with associated circuitry and other electro-optical elements. They are being used in areas such as data communication, optical storage and optical beam printing.
An extensive range of different laser structures has been suggested and is being used. One of the fundamentally simplest and most reliable laser devices available today is the self-aligned ridge laser. One such laser and its fabrication process has been described in an article "High Power Ridge-Waveguide AlGaAs GRINSCH Laser Diode" by C. Harder et al (published in Electronics Letters, 25th September 1986, Vol. 22, No. 20, pp. 1081-82).
In the past, most of the efforts made in designing semiconductor lasers were directed to GaAs-system devices operating at a wavelength of about 0.8 .mu.m. However, particularly for communications applications, lasers emitting beams of longer wavelength (in the order of 1.3 .mu.m) are also in great demand since they better match the transmission characteristics of the optical fiber links often used. An extensive survey on such structures, including ridge waveguide lasers, and their performance is given in chapter 5 of a book entitled "Long-Wavelength Semiconductor Lasers" by G. P. Agrawal and N. K. Dutta (Van Nostrand Reinhold Company, NY).
In the fabrication of self-aligned ridge laser structures, normally a single photolithographic mask is used to define the entire contact/ridge geometry area throughout the whole ridge formation process. However, problems arise when this process is applied to long-wavelength, InP-system lasers. Severe undercutting during etching occurs at the photoresist/GaInAs contact interface which significantly reduces the ohmic contact area whereby the overall contact resistance is increased. This, in turn, leads to increased heating of the laser with a negative impact on device properties.
A number of authors have investigated the reasons for and the consequences of the photoresist etch mask undercut problem. Some of their articles are given below:
"Preferential Etching of InP Through Photoresist Masks" by D. T. C. Hus et al (J. Electrochem.Soc.: Solid-State Science and Technology, Vol. 135, No. 9, pp. 2334-2338); PA1 "Performance of an improved InGaAsP Ridge Waveguide Laser at 1.3 .mu.m" by I. P. Kaminow et al (Electronic Letters, 30th April 1981, Vol. 17, No. 9, pp. 318-320); PA1 "On the Formation of Planar-Etched Facets in GaInAsP/InP Double Heterostructures" by L. A. Coldren et al (J. Electrochem.Soc.: Solid State Science and Technology, Vol. 130, No. 9, pp. 1918-1926).
It has been found that the problem of underetching is at least partly due to inefficient adhesion of the photoresist mask to the contact layer. Although a variety of adhesion promotion techniques are known in semiconductor technologies (ranging from the use of adhesion promotion films, special mask treatments or materials, to the use of specific etchants or etch processes), they have not yet led to a satisfactory solution for the fabrication of long wavelength ridge waveguide lasers. The overall task of providing a simple, workable and reproducible ridge formation process offering an adequate "process window" for realistic fabrication tolerances, is furthermore complicated in that, in the sequence of the process steps using the same photoresist mask, a sufficient etch selectivity, (i.e., the selective controlled removal of one material over the other), needs to be achieved to allow the fabrication of the required ridge profile. The problem is not so much to find solutions for each of the specific problems, but to provide an overall concept that solves the problems, that are not isolated from each other, as a whole. To our knowledge, no process satisfying the numerous requirements has yet been presented.